1. Field of the Invention
The present invention relates to a method for producing an n-type Group III nitride-based compound semiconductor, including crystal growth carried out by the flux process employing a flux; and to an n-type Group III nitride-based compound semiconductor produced by the method.
2. Background Art
According to a conventional sodium (Na) flux process, in which nitrogen gas is introduced into a gallium melt prepared by melting metallic gallium by use of Na serving as a flux, to thereby grow a gallium nitride crystal, an impurity-undoped GaN single crystal can be grown at a pressure of about 5 MPa and at a relatively low temperature of 600° C. to 800° C.
As disclosed in, for example, the following patent documents: Japanese Patent Application Laid-Open (kokai) Nos. H11-060394, 2001-058900, 2001-064097, 2004-292286, 2004-300024, and 2007-277055; Japanese Patent Nos. 4030125 and 4001170; and WO 2004/067814 and WO 2007/83768 in conventional methods for producing a Group III nitride-based compound semiconductor crystal, the crystal is grown by the flux process. Such a conventional production method generally employs, as a seed crystal, a template substrate including a sapphire substrate, a buffer layer vapor-grown thereon, and a Group III nitride-based compound semiconductor vapor-grown on the buffer layer; a GaN single-crystal self-standing substrate; or a similar substrate.
Japanese Patent Application Laid-Open (kokai) No. 2004-300024 discloses a method for growing GaN by the flux process, in which impurity doping is carried out by supplying a gas such as SiH4 or GeH4. Japanese Patent Application Laid-Open (kokai) No. 2007-277055, Japanese Patent Nos. 4030125 and 4001170, and WO 2004/067814 and WO 2007/83768 suggest that germanium may be used as a donor in the flux process. Japanese Patent Application Laid-Open (kokai) No. 2007-277055 and Japanese Patent No. 4001170 also suggest that carbon may be used as a donor in the flux process. However, neither Japanese Patent Application Laid-Open (kokai) No. 2007-277055 nor Japanese Patent No. 4001170 suggests use of carbon as a catalyst, rather than as a donor, for effectively incorporating germanium into a grown semiconductor crystal. None of the aforementioned patent documents suggests that the resistivity of a grown semiconductor crystal can be controlled with germanium density.
In the conventional Na flux process, when an impurity element is dissolved in a flux, difficulty is encountered in producing a donor-doped n-type semiconductor crystal having a sufficiently high concentration of electrons and exhibiting high crystallinity. For example, when the flux process is carried out by use of dissolved silicon (Si) and gallium (Ga) serving as donors for the production of n-type GaN having an electron concentration of about 1017/cm3 to about 1020/cm3, crystal growth of gallium nitride fails to be achieved. Similarly, in the case of addition of germanium (Ge), n-type GaN fails to be grown by the flux process by use of a melt containing dissolved Ge and Ga.